1. Field of the Invention
The present invention relates to the regeneration of catalysts employed in a fluid catalytic cracking process. More particularly, this invention relates to the introduction of spent catalyst into a regeneration zone by means of a catalyst distribution system comprising stationary distributor arms. A further aspect of the invention relates to the discharge of regenerated catalyst from the regeneration zone by means of a collection system comprising a shrouded catalyst outlet.
2. Description of the Prior Art
The fluidized catalytic cracking of hydrocarbons is well known in the prior art and may be accomplished by a variety of processes which employ fluidized solid techniques. Normally in such processes, preheated, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely-divided, solid catalyst particles either in a fluidized bed reaction zone or in an elongated riser reaction zone. The mixture of hydrocarbons and catalyst is maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the reaction zone, thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a portion, preferably a major portion, of the activity to the coke contaminated or spent catalyst, the catalyst is transferred from the reaction zone into a regeneration zone. A typical regeneration zone comprises a large vertical substantially cylindrical vessel wherein the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas such as air. The fluidized catalyst forms a dense phase catalyst bed in the lower portion of the vessel and a dilute catalyst phase containing entrained catalyst particles above, with an interface existing between the two phases. The catalyst is contacted with the oxygen-containing regeneration gas under conditions sufficient to burn at least a portion, preferably a major portion, of the coke from the catalyst. Flue gas, which normally comprises gases arising from the combustion of the coke on the spent catalyst, inert gases such as nitrogen from air, any unconverted oxygen, and entrained catalyst particles, are then passed from the dilute catalyst phase into solid-gas separators within the regeneration zone (e.g., cyclone separators) to prevent excessive losses of the entrained catalyst particles. The catalyst particles separated from the flue gas are returned to the dense phase catalyst bed. The regenerated catalyst is subsequently withdrawn from the regeneration zone and re-introduced into the reaction zone for reaction with additional hydrocarbon feed.
For maximum process efficiency, it is important that there should be uniform dispersal and distribution of the spent catalyst entering the fluidized bed of the FCCU regenerator. For example, dead catalyst zones will cause deactivation of the catalyst. On the other hand, if the catalyst exits the regenerator by a shortcut before being adequately dispersed, it will experience incomplete regeneration. It has been estimated that several pecent, for example 2 to 5 percent, of the catalyst typically short-circuits the regenerator bed and is therefore not sufficiently regenerated.
In the prior art, spent catalyst typically enters the regenerator vessel through a riser, or vertical conduit, which terminates just above a grid. It is known to deflect the incoming catalyst flow by means of a flat plate above the riser outlet. However, there is usually no positive distribution of the incoming catalyst throughout the bed. As a result of incomplete distribution, concentration gradients of the gases leaving the regenerator bed are produced and localized afterburning, in the cyclones above the riser or above the overflow well, may occur.
It has been the practice to merely correct the symptom of afterburning, by providing more holes in the regenerator grid near the spent catalyst entrance. In some units 55% of the regenerator grid holes are in the side of the grid near the riser. However, this unequal hole distribution results in bed velocities much higher in this area, particularly when taking into account the 10% regenerator air entering with the spent catalyst. These high gas velocities cause shorter gas residence time in the bed in this area. They also cause pronounced gulfstreaming of the catalyst in the bed, which brings high-carbon partially regenerated catalyst to the top of the bed where it can short-circuit to the overflow well outlet. The high gas velocities also increase entrainment in that area of the regenerator vessel. This is a particular debit to a unit running at maximum velocity as limited by entrainment. Thus, there would be a capacity credit for even distribution. Of course, if there is uniform gas composition leaving the dense phase fluidized bed, then there would be a much reduced chance for afterburning, thus correcting the cause of the problem rather than the symptom.
It is also conventional to employ an overflow well to collect regenerated catalyst from the regenerator bed for return to the reaction zone. Such an overflow well also provides for control of bed height, particularly if gas/solids in-flow perturbations occur. However, the conventional overflow well is subject to certain problems. Typically, in a fluidized bed such as in the regenerator, both dense phase solids and also regions of lower density "bubbles" rise through the bed, with phase disengagement occurring at the top of the bed. It is known that the bubbles rising through the regenerator bed, 95% of gas flow, have a particle wake and tend to drag catalyst to the top of the bed very quickly. The catalyst particles then gradually migrate downward in the bed. Thus, a conventional overflow well can be fed by high-carbon spent catalyst that has been brought to the top by the bubbles. It is known to shield the overflow well or catalyst outlet from the top of the bed by a shroud, so to prevent somewhat this bypassing. Preferably, the entry to the space above the overflow well is selected to provide the lowest-carbon catalyst, and the top of the shroud is restricted to reduce the amount of entrained catalyst entering. Prior art shrouds are shown in commonly assigned U.S. Pat. Nos. 3,902,990 and 3,958,953.
Solids distributors have been used outside the field of the invention. For example, U.S. Pat. No. 3,784,108 discloses the use of equal rotating radial arms for distributing solids in a fluid bed combustor. Similarly, U.S. patent teaches a rotating radial arm feed device. These prior art devices have arms that rotate, whereas the subject invention employs a plurality of stationary arms. The Kellogg Company is believed to employ spent catalyst distributors in their catalytic cracking units, although the specific details are not public to applicant's knowledge.
An object of the present invention is therefore to provide an apparatus for maximizing the dispersion and distribution of the catalyst in the regeneration zone of a fluid catalytic cracking unit, so to prevent both catalyst dead spots and by-passing of the bed, and to thereby achieve sufficient residence time for the desired regeneration of spent catalyst.